This invention relates to the treatment of pyrophoric materials such as sponge iron.
Sponge iron is utilized in the steel making industry as a basic raw material source for the production of steel. Generally speaking, sponge iron is produced by exposing hematite (Fe.sub.2 O.sub.3) iron ore in comminuted form to a reducing gas environment at temperatures somewhat below blast furnace temperatures. The production of sponge iron is the subject of a large number of patents, including the following U.S. Pat. Nos.: 2,243,110; 2,793,946; 2,807,535; 2,900,247; 2,915,379; 3,128,174; 3,136,623; 3,136,624; 3,135,625; 3,375,098; 3,423,201; 3,684,386; 3,765,872; 3,770,421; 3,779,741; 3,816,102; 3,827,879; 3,890,142; and, 3,904,397. The final sponge iron product of practically all of the processes disclosed in these patents is in a particulate or pellet form.
Typically, the components of sponge iron are metallic iron, iron oxide, gangue and possibly carbon. Metallic iron is iron which has been totally reduced by the reducing gas environment. Gangue is the term used in the industry to refer to all non-ferrous material, except carbon contained in the ore. Gangue may include silica, alumnia, lime, magnesia, phosphorus, sulfur and possibly other materials. A deposit of carbon on the outside surface of the sponge iron particulate will be described in greater detail hereinafter. In all of the iron ore reduction processes just referred to, freshly produced sponge iron as found in the final step of the process may be at a temperature of 300.degree. F. or, in some cases, significantly higher. The freshly produced sponge iron must be moved from the reactor to some type of storage location or be immediately utilized in a steel producing process. In the past, it was more typical that the freshly produced sponge iron be used rather quickly in the production of steel. However, in the last few years, this situation has changed. There are more and more iron ore reducing plants being built in various parts of the world entirely removed from steel producing facilities. Therefore, it has become necessary that sponge iron be stored and even shipped long distances.
Freshly produced sponge iron is not a stable material. In fact, such sponge iron is pyrophoric and subject to degradation through oxidation by exposure to air or water.
There are two mechanisms by which sponge iron is believed to reoxidize. In the first mechanism sponge iron will react with dry air--i.e. oxygen--to form a magnetite by the following reaction: EQU 3Fe+2O.sub.2 .fwdarw.Fe.sub.3 O.sub.4 ( 1)
This reaction is very exothermic and can generate enough heat to spontaneously ignite adjacent sponge iron particles. Reoxidization presents a particularily aggravated problem when the sponge iron is already at high temperatures such as when it is removed from the reduction furnace.
The problem of pacifying freshly produced sponge iron against reoxidation conventionally involves cooling it to a safe temperature.
Attempts to at least partially cool the sponge iron to a safe temperature are found in the prior art. It is known that freshly reduced sponge iron must be cooled down significantly. Some cooling has been incorporated into the reduction process. Generally, this initial cooling occurs while the sponge iron is still in the reduction reactor. U.S. Pat. No. 3,904,397 of Celada and others discloses the utilization of cooled, spent reducing gas in such a cooling reactor. Other U.S. patents which refer generally to the utilization of a cooling step immediately after reduction include U.S. Pat. Nos. 3,765,872; 3,684,486; 3,136,625; 3,136,624; and, 3,136,623. Recently, I have discovered and disclosed a separation and cooling process by which sponge iron may be better pacified against reoxidation. A description of this new process is found in my U.S. Pat. No. 4,169,533.
A second mechanism by which sponge iron is believed to reoxidize involves its reaction with water vapor and air. This process is referred to as "rusting" and proceeds by a two stage reaction as follows: EQU 2Fe+2H.sub.2 O+O.sub.2 .fwdarw.2Fe(OH).sub.2 ( 2) EQU 2Fe(OH).sub.2 +H.sub.2 O+1/2O.sub.2 .fwdarw.Fe.sub.2 O.sub.3 +3H.sub.2 O(3)
The hydrated ferric oxide formed by reaction (3) may undergo yet another reaction: EQU 3Fe(OH).sub.2 .fwdarw.Fe.sub.3 O.sub.4 +H.sub.2 ( 3)
to liberate water and hydrogen.
The "rusting" process not only results in the loss of pure iron which presents a serious economic loss over long periods of storage--estimated loss to rust may be as high as 1.5% by weight per month--it also presents a dangerous shipping problem since hydrogen is generated as a by-product of the rusting reactions.
Since in many cases sponge iron facilities are located at great distances from steel mills, and often must be transported to such distant mills by sea transport, some method must be found by which its tendency to rapidly reoxidize upon contact with moisture in the air, with the consequential liberation of heat and hydrogen, can be eliminated or significantly reduced.
Although sponge iron treated in accordance with the process disclosed in my U.S. Pat. No. 4,169,533 has greatly reduced reoxidation tendencies--whether contacted by air alone or the combination of air and water vapor--the tendency to reoxidize upon contact with moisture may still persist to an undesirable degree.
Ideally, the reoxidation problem could be overcome if the sponge iron could be contacted with a substance which would coat and shield its active metal surfaces from contact with air or moisture or otherwise inhibit the occurence of the oxidation reactions at such surfaces.
Several coating methods have been disclosed in the prior art. U.S. Pat. Nos. 3,816,102 of Celada et al. and 3,136,624 of Mader et al. disclose a process for coating or depositing a layer of carbon onto the hot sponge iron during the initial cooling of the just-reduced sponge iron. Carbon is deposited for the next step in the process, i.e. an electric furnace, which converts the iron to steel, and also the carbon present reacts with the remaining iron oxide to finish the reduction (FeO+C.fwdarw.CO+Fe). One of the results of the deposition of the carbon layer on the sponge iron is the formation of a protective exterior shell against reoxidation of the hot sponge iron because iron combined with carbon, such as Fe.sub.3 C, is supposedly less sensitive to oxygenation than the reduced metallic sponge iron. "Storage and Transportation of HYL DRI Pellets" presented by Ing. Raul G. Quintero, Hylsa, S.A. and Mr. G. E. McCombs, Pullman Swindell, Third Direct Reduction Congress, Instituto Latinoamericano del Fierro y el Acero, Caracas, Venezuela, July, 1977. U.S. Pat. No. 3,423,201 of Celada et al. discloses a method for cooling sponge iron having such a carbon layer deposited thereon. In Celada U.S. Pat. No. 3,423,201, a second cooling step is initiated when the temperature of the reduced ferrous material in the cooling reactor has dropped below the value at which cracking of reducing gas (and thus depositing of carbon on the sponge iron particulate) occurs. The Celada U.S. Pat. No. 3,423,201 states that the sponge iron is cooled to a temperature "near room temperature".
Another recent coating process for rust prevention was disclosed by U.S. Pat. No. 4,069,015, wherein sponge iron is immersed in an aqueous solution of a water soluble alkali metal silicate. As discussed therein many other coating materials have been suggested in the prior art, such as asphalt, plastics, and waxes.
Another proposed solution to this problem has been suggested by the Midrex Corporation. Midrex Corporation has made public a chemical treating process sold under the trademark CHEMAIRE. The CHEMAIRE process is a combination of chemical treatment and air passivation to inhibit rusting and reoxidation. "Direct From Midrex", Vol. 3, No. 2 brochure. Disadvantages of this type of system are several. First of all, the complete distribution of the chemical upon the particulte sponge iron is very unlikely. Secondly, the addition of the chemicals may or may not have any effect upon subsequent use of the sponge iron in the production of steel.
Although many types of protective coatings have been suggested none have proved very satisfactory when applied to sponge iron. In part this has been due to their expense, the difficulty in applying them to the sponge iron and their subsequent contaminating effect upon any steel which may be produced from a coated sponge iron.
Additionally, since sponge iron is a very porous material and thus has active or oxidation prone surfaces on both the exterior and the interior of the pellets any coating to be fully effective must be applied to all such surfaces. Coating suggested in the prior art for the most part lack the fluidity necessary to penetrate through the complex of minute pores of the sponge iron pellet and thus provide no protective coating on its interior active metal surfaces. Since prior art coating exist only on the pellets' exterior surface it may easily be damaged during storage or loading processes thus removing it in those areas wherein the pellets are contacted by other pellets or objects. Once the coating is removed the uncoated interior active metal surfaces are fully exposed to air and moisture and hence are susceptible to reoxidation.
For any sponge iron coating or inhibiting process to be a viable solution to the reoxidation problem the substance which is applied to coat or inhibit the sponge iron, it must be inexpensive, easy to apply, be a non-contaminate to the steel making process and be capable of application to both the exterior and interior active metal surface of the sponge iron. Desirably, any such coating or inhibiting material should be capable of application to the sponge iron by adding it to a cooling gas which is used to reduce the temperature of such sponge iron to a safe level.
As yet, no completely satisfactory coating or inhibiting material has been suggested or disclosed in the prior art. Additionally, no gas cooling process extant prior to that disclosed in my U.S. Pat. No. 4,169,533 has been suggested or disclosed which would insure that a suitable coating or inhibiting material, once found, could be thoroughly and completely contacted with a bulk mass of sponge iron so as to insure that substantially all pellets thereof are equally treated with such material.
Basically, all of the prior sponge iron cooling processes disclose the cooling of the sponge iron while still in a reactor. In order to cool the sponge iron in a reactor, it is necessary for the cooling gas to flow through a bulk mass, or pile of sponge iron. Typically, the cooling gas takes the paths of least resistance and therefore is not equally distributed among all the sponge iron particulate. Further, the cooling gas serves to deposit fines in particular locations out of the flow paths of direct cooling gas flow so that hot spots of fines are formed. Such fines may also obstruct flow paths through the particulate and thus prevent cooling. Hence any protective coating or inhibiting material which is added to the cooling gas would not equally contact all pellets of the sponge iron mass.
In summary, no completely suitable coating or inhibiting material has been disclosed which is satisfactory from the standpoint of cost, ease of application, interior-exterior coating properties and which is non-contaminating to subsequent steel manufacturing. Also, no conventional cooling process has been disclosed by which a suitable coating or inhibiting material may be completely and thoroughly contacted with a bulk mass of sponge iron pellets whereby substantially total coating or inhibiting of all of the bulk mass may be achieved simultaneously with cooling of the sponge iron.